Even those in the 'backroom' (like me) are typically under some fairly agressive non-disclosures with the VC themselves to avoid leakage of company info. That won't stop jerks from trying to pump people for information but I have yet to see someone get any mileage out of that. I have seen a reputation or two being destroyed that way (of the person trying to get information in such a back-handed manner).

VCs are very image conscious, if they lost their influx of potentials because their reputation took a hit because of a real or perceived hi-jack of someone else's idea then they would suffer immensely so they are very protective of this channel.

I'm not sure if it would be an existential issue but I'm definitely sure that it would hurt badly to be known as 'the VC that you can't pitch to because they'll rip your idea'.